The present invention relates to image generation techniques for electronic displays. More particularly, the invention relates to programmable interlace with optional skipping of non-information carrying lines in a raster-scanned display, to a related contrast enhancement technique herein termed "rolling writing", and to direct point writing techniques. While techniques of the invention are applicable to any long-persistence display, the specific example disclosed in detail herein is a cathodochromic CRT display, which has an infinitely long persistence, until it is deliberately erased.
Most CRT based display systems, as well as some flat panel displays, use the raster scan technique of image generation. Raster scan in a CRT employs a scanning electron beam to write an image on the phosphor-coated face of the CRT a line at a time from top to bottom. In CRTs with short duration phosphors, like a television set, this process is repeated at fairly high rates, 30 to 60 times per second. The high rate is needed to reduce flicker in the image caused by the fading of the light emitted by the relatively short persistence phosphor after it is hit by the scanning electron beam. Longer duration phosphers are not used in conventional television because they would preclude motion in the generated image.
If all lines of the image are scanned sequentially in one pass from top to bottom, the generated image by definition has a 1:1 interlace ratio. If the image is generated in two passes, one pass doing the odd lines and the other doing the even lines, then by definition the generated image has a 2:1 interlace ratio. The entire generated image is called a frame. If a 2:1 interlace, or higher, is used, each pass is called a field and the display must show "n" fields to make a complete frame, where "n" is the interlace ratio. Interlace is used to reduce flicker by increasing the apparent refresh rate and lower the bandwidth requirements of the display electronics. U.S. standard television uses a 2:1 interlace. Interlace ratios higher than 2:1 are very rarely used in displays, and 1:1 is used in most high resolution applications.
In storage and long persistence displays higher interlace factors can be useful. Since the images used are static, a detailed high resolution image can be generated at higher interlace factors providing a low resolution image very quickly. The image then becomes more detailed as the subsequent fields are scanned.
As noted above, while the invention is applicable to any long-persistence display system, the particular application described herein is a high resolution cathodochromic CRT projection display system. Accordingly, several characteristics of such a display will now be summarized.
An image target on which an electron beam impinges in a cathodochromic CRT does not emit light as does an image target in a cathodoluminescent CRT. Rather, the cathodochromic materials employed change color when excited by an electron beam. In the case of an image target comprising cathodochromic bromine sodalite, the resultant coloration remains indefinitely, until deliberately erased. In addition to inherent memory, cathodochromic image targets have the properties of high resolution, and high contrast in bright ambient light making them highly suitable for projection systems.
Erasure of a cathodochromic image target is normally effected by heating to about 300.degree. C. An economical and technically feasible erasure method is electron beam heating, wherein the image target is scanned, in a raster pattern, with an electron beam spot energy density such that temperature is raised above an erase threshold.
Processes for preparing cathodochromic sodalite and a cathodochromic CRT projection display are disclosed in Todd, Jr. et al U.S. Pat. No. 3,932,592 and Todd, Jr. U.S. Pat. No. 3,959,584 to which reference may be had for further details.
Considering cathodochromic CRT display characteristics in greater detail as they relate to the present invention, the darkness of the black pixels is a function of electron beam exposure time, electron beam current level, and the temperatur of that pixel. The longer the exposure, the darker the pixel gets, so long as the temperature of the cathodochromic material at that pixel site remains under the erase threshold temperature. A problem with long exposure times is that the material is heated by the electron beam to the point where it loses contrast (erases). This problem is exacerbated by the use of a thermal insulation or buffer layer between the sensitized cathodochromic material and the underlying support, as is disclosed in the above-referenced Todd, Jr. U.S. Pat. No. 3,959,584. The thermal buffer layer aids in electron beam erasure, but does complicate the writing process if good contrast is to be achieved. Thus, multiple short exposures of a high current electron beam separated by a relatively long cooling period are necessary for building good contrast. Typically, from 40 to 300 exposures per pixel may be employed in a raster-scanned display before the final contrast is achieved.
As a simple example, a 100 nanosecond exposure with a delay of 100 miliseconds before the next exposure is fairly typical. This provides a cooling delay of 1,000,000 times the exposure time. Carrying this example further, if the display comprises 1,000 lines of 1,000 pixels each, a conventional raster scan would ideally expose each pixel once every 100 milliseconds (1,000,000 pixels.times.100 nanoseconds per pixel), giving a frame refresh rate of 10 Hz (1/100 ms). Various system constraints, however, could reduce the frame refresh rate to as low as 3 Hz for such a high-resolution display.
This leads to a closely related problem, known as flicker. A frame refresh rate of 10 Hz in a display employing conventional (short persistence) phosphor would have an unacceptably annoying flicker. Interlace is commonly used to provide an acceptable flicker level in a display system that has reduced cost due to slower components, i.e the horizontal scan time is roughly doubled in going to a 60 Hz field rate from a 60 Hz frame rate. This can greatly reduce the display system cost and reduces overall system bandwidth requirements, and also results in a 50% (roughly) reduction in transmission spectrum space for commercial TV over a 60 Hz system. Typical displays either sync or operate the field/frame rates at the same (or multiple/sub multiple) as the power line frequencey (60 Hz in the U.S.) to reduce artifacts. Such is the case with standard television, where each of two alternating fields (odd numbered then even numbered lines) is scanned at 60 Hz, yielding a 30 Hz frame refresh rate and resulting in barely perceptable flicker.
The manner in which these problems relate to each other in a high resolution cathodochromic CRT display will now be considered. As noted above, from 40 to 300 exposures per pixel may be required to build contrast. Correspondingly, multiple complete frame exposures allow the image to build contrast over an interval measured in seconds. At the beginning of the process of generating an image, particularly up until the point where the contrast ratio is about 2:1, the contribution to contrast of each successive frame exposure is quite noticeable to the extent that even an infinitely-long persistence cathodochromic CRT has a perceptable flicker, much as a conventional phospher CRT would have at the same frame rate. In a cathodochromic CRT, the flicker effect is manifested in part as an annoyingly visible top to bottom contrast enhancement.
Interlacing can also alleviate the flicker problem in a cathodochromic CRT. With an interlace ratio such that the resultant field rate approaches 30 Hz, the contrast builds and an image gradually appears much as the image on an "instant" photograph appears as it develops.
Another advantage of interlacing is that the heating caused by thermal conduction from adjacent pixels on lines above and below occurs at least one field time (rather than one line time) away, thus reducing the peak temperature of the exposed pixels allowing contrast to build faster and darker.
Interlacing alone does have its limitations in a high resolution cathodochromic CRT display system, due largely to the time required for a complete scan. As a more particular example, in one system in which the present invention is embodied, there are a total of 2048 lines on the display, with 1728 pixels per line. At an exposure of 100 nanoseconds per pixel, multiplication gives an active line time of 172.8 microseconds. Rounding this to 172 microseconds and adding a 10 microsecond horizontal retrace time results in a total time of 180 microseconds per line. Multiplying again by the 2048 lines gives a frame period of 0.369 seconds, which corresponds to a frame refresh rate of 2.7 Hz. This figure is even worse when time for vertical retrace and possible calculations during the vertical retrace interval is taken into account.
To achieve a field refresh rate of 30 Hz under these conditions would require an interlace ratio of about 12:1. At such high interlace ratios other objectional effects can occur, such as an apparently random appearance of spaced lines on the display, instead of the desired effect of having an image gradually appear much as an "instant" photograph.
Moreover, interlacing alone does nothing to speed the overall process of generating an image over 40 to 300 frames, and in fact can slow the process down by introducing multiple vertical retrace delays
Related problems arise in two other situations with which the invention is particularly concerned.
The first of these situations is where only partial raster images are available, such as from a line-by-line facsimile transmission. In general, an image builds from top to bottom. However, due to overheating and contrast considerations, lines cannot simply be written to the image target as they are received.
The second of these situations is when manual point-by-point line drawing is implemented to allow an image to be annotated. Overheating and contrast considerations remain, complicated by the random nature of the input.